[Electrical force] 2 balls hanging from ceiling, angled

[haruspex]

I still don't see how this is correct as angle 10 is not adjacent to r

Draw a vertical line down from the apex and consider the two triangles produced.

 

[sea333]

I don't think I will need these advance techniques in my exercises

 

[haruspex]

I don't think I will need these advance techniques in my exercises

Dropping a perpendicular from the apex of an isosceles triangle to create two congruent right angled triangles is pretty basic geometry; less advanced, even, than the cosine rule.

 

[sea333]

Dropping a perpendicular from the apex of an isosceles triangle to create two congruent right angled triangles is pretty basic geometry; less advanced, even, than the cosine rule.

I have checked this and I get r = 2a*sin10

 

[haruspex]

I have checked this and I get r = 2a*sin10

Yes, as in my correction in post #27.

 

[sea333]

sqrt(4a^2sin^2(10)) = 2*a*sin10

How do you get from 1 - cos^2(10) - sin^2(10) to 2 sin^2(10) ?

1 - cos^2(10) + sin^2(10) = 2sin^10 because cos^2(10) + sin^2(10) = 1